So, before I go any further, I need to clarify that I'm trying to pixel-count directly on my TV because I don't have a good capture card, and it's painful and my head is spinning and being precise without physically touching the screen is hard.
But, I tried running Halo's 3 and ODST with my 360 set to 480p via YPbPr component, and I'm counting a stairstep
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
ixel ratio in the vague neighborhood of 7:12, which would imply 630p (1080*7/12 = 630). It seems clearly higher than 1:2, and it would have to be below 1:2 in order to be 480p. It would be nice if someone with a decent capture card could confirm, but what I'm seeing suggests that the game is likely still natively rendered at 640p.
IMO it's not surprising that the game would still "look like it has no AA." On the edges where aliases are most noticeable (near-horizontal and near-vertical), at low orders, ordered-grid sampling gives results of a quality comparable to roughly the square root of similar sparse-grid patterns; it takes about a 4xOGSSAA to deal with those aliases as effectively as a 2xSGSSAA. And since we're only looking at ~1.8xOGSSAA if the low-res buffer is 853x480 16:9, by this approximation the effectiveness of the AA on near-horizontal and near-vertical aliases will be comparable to a hypothetical "1.34xSGSSAA"; While noticeable and beneficial (including in ways other than decreasing edge jaggies), the most obvious sorts of aliases aren't going to be dealt with all
that effectively.
This is exacerbated by Halo 3's tendency to use very high-frequency details. Between the increased geometric complexity and the amount of shader aliasing due to unfiltered normal maps, I always thought that Halo 3 suffered from significantly worse aliasing than the original Xbox games, despite the latter running at a lower resolution. I have to suspect that, even if it "looks like it has no AA", it's probably still a significant improvement over what Halo 3 would look like at actual 480p.
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tl;dr Halo 3 still seems to be running at 640p (resulting in OGSSAA) when the 360 is in 480p mode